The present invention comprises an improvement in the energy economizing AC power control system described in Hedges U.S. Pat. application Ser. No. 165,574, filed July 3, 1980, now U.S. Pat. No. 4,297,628 issued Oct. 27, 1981, for "Energy Economizer for Induction Motors", the disclosure of which is incorporated herein by reference.
When a conventional induction motor is operating below its full rated load or under conditions of higher than normal stator voltage, a fraction of the sine wave voltage would satisfy the actual load requirement imposed on the motor. Such cutting in part of the sine wave voltage would result in considerably less iron and copper losses and less heating of the stator. The resultant low operating temperature further reduces the copper losses in the motor due to lowered ohmic resistance. These factors combine to effect a significant reduction in the energy which is consumed by the induction motor, with a consequent conservation in available energy sources and reduction in motor operating costs.
The energy economizer of the aforementioned Hedges Patent application is based upon a recognition of the foregoing factors, and provides a simple yet reliable mechanism operative to cause the electrical energy supplied to the stator and the stator flux density of a standard-unmodified-AC induction motor to become a function of its operating efficiency or load demand at any given moment. The prior Hedges invention accomplishes this by permitting a greater or smaller portion of the sine wave of voltage from a power source to enter the stator as a function of operating efficiency-related characteristics of stator inrush current during each alternation. In other words, the sine wave of the voltage supplied to the motor's stator is modified to suit existing load and AC power source conditions. This results in the reduction of iron and copper losses.
The prior Hedges invention, however, affords no protection to a controlled motor under adverse conditions of excessive overloads or temperature and, additionally, may actually decrease reliability of motor system operation. For example, substantially higher stator current under said conditions severely overloads an economically-sized solid state AC power switch, e.g., a Triac, with adequate power rating for normal motor operation, before causing permanent damage to the motor. Using an over-sized AC power switch to accommodate excessive stator current would be a cost and space penalty that merely guarantees the referenced energy-saving invention could couple sufficient power to permanently damage the motor under, for example, a stalled rotor condition. The present invention increases motor system reliability by preventing catastrophic failure of the motor or an economically-sized solid state switch under said adverse conditions at a lesser cost than would be required for an oversized AC power switch assembly.
The improvement of the present invention is characterized by increased stability and function of the motor efficiency/load demand monitoring means disclosed in the aforementioned Hedges patent, and the addition of a motor protector circuit. The motor protector utilizes a stator current-related signal developed by the improved monitoring means to activate low-cost solid state means which inhibits application of stator input power under stalled-rotor or severe overload conditions; this reliably protects both the motor and the electronic energy-saving control means. Additionally, said protector means may be utilized to further protect the motor by likewise inhibiting application of stator input power when associated temperature sensors in physical contact with the controlled motor detect a condition of motor heating above a preselected temperature. As will be described, the present invention realizes the energy-saving results of the prior Hedges invention under more adverse circumstances and with improved stability under all conditions, prevents catastrophic motor system failure due to excessive stator current and, further, affords additional motor protection by automatically removing stator power if motor temperature rises above a preset, safe operating level for any reason (e.g., poor ventilation).